A G₁ Checkpoint Mediated by the Retinoblastoma Protein That Is Dispensable in Terminal Differentiation but Essential for Senescence

Abstract

Terminally differentiated cell types are needed to live and function in a postmitotic state for a lifetime. Cellular senescence is another type of permanent arrest that blocks the proliferation of cells in response to genotoxic stress. Here we show that the retinoblastoma protein (pRB) uses a mechanism to block DNA replication in senescence that is distinct from its role in permanent cell cycle exit associated with terminal differentiation. Our work demonstrates that a subtle mutation in pRB that cripples its ability to interact with chromatin regulators impairs heterochromatinization and repression of E2F-responsive promoters during senescence. In contrast, terminally differentiated nerve and muscle cells bearing the same mutation fully exit the cell cycle and block E2F-responsive gene expression by a different mechanism. Remarkably, this reveals that pRB recruits chromatin regulators primarily to engage a stress-responsive G₁ arrest program.

We have no conflict of interest with the publication of this research.

We are grateful to A. Chicas and S. Lowe for freely discussing unpublished work. We thank many colleagues at the LRCP and CHRI for advice during the course of this work, especially Jai Ablack for providing Ad-GFP. The Ad-E2F1 virus was a kind gift of E. Knudsen (Kimmel, Philadelphia, PA). We are grateful for technical assistance by the CHRI histology, LHSC histology, and LHSC flow cytometry core facilities.

S.T. is supported by an ERA award. C.E.I., S.A.H., and S.M.F. have all been members of the CIHR strategic training program in cancer research. S.M.F. acknowledges fellowship support from CBCF Ontario and OGSST, and S.A.H. thanks CBCF Ontario for postdoctoral fellowship support. F.A.D. is a research scientist of the Canadian Cancer Society. This work was supported by operating grants from the CIHR to R.B. (MOP-64243) and to F.A.D. (MOP-64253).